In a conventional gas turbine engine, gases, e.g. atmospheric air, are compressed in a compressor section of the engine and then flowed to a combustion section in which fuel is added, mixed and burned. The now high energy combustion gases are then guided to a turbine section where the energy is extracted and applied to generate a rotational movement of a shaft. The turbine section includes a number of alternate rows of non-rotational stator vanes and moveable rotor blades. Each row of stator vanes directs the combustion gases to a preferred angle of entry into the downstream row of rotor blades. The rows of rotor blades in turn will carry out a rotational movement resulting in revolving of at least one shaft which may drive a rotor within the compressor section and/or a generator.
A known nozzle guide vane assembly of a turbine section of a gas turbine engine may comprise a circumferentially extending array of angularly spaced apart aerofoils. Inner and outer platform members are separate from the aerofoils and each platform members may comprise an inner and outer skin. The skins may have aerofoil shaped apertures through which the aerofoils project. The inner skin serves to define a respective boundary of the gas flow through the assembly. The outer skin may be provided with a large number of impingement cooling apertures as high temperatures may occur within the turbine section. By causing cooling fluid at high pressure to flow through these apertures and to impinge upon the inner skin an efficient cooling of the inner skin may be provided. A nozzle guide vane like this is defined in U.S. Pat. No. 4,300,868.
The reason for cooling is that due to the very high temperatures in the turbine flow duct. The surface of the platform exposed to the hot gas is subjected to severe thermal effects. In order to cool the platform, a perforated wall element may be arranged in front of the surface of the platform facing away from the hot gas. Cooling air enters via the holes in the wall element and hits the surface of the platform facing away from the hot gas. This achieves efficient impingement cooling of the platform material.
Besides the platforms, it is common also to cool aerofoils, e.g. by injecting cooling air into a hollow interior of an aerofoil.
A ring of guide vanes may be arranged by a plurality of guide vane segments. A segment comprising the inner platform, the outer platform and at least one aerofoil may be cast as a single piece. A plate for impingement as a separate piece may later be assembled to the cast segment.
Alternatively, according to U.S. Pat. No. 6,632,070 B1, also the platform may comprise several pieces. The platform may have a so called separating region, which is embodied as a separate component. The separating region may be arranged with a plurality of cooling pockets, covered by an impingement cooling sheet with impingement cooling openings, such that cooling air jets can hit the surface of the cooling pockets.
A further implementation showing cooling pockets in which impingement cooling takes place and from which the cooling air is guided away via film cooling holes is disclosed in FR 2 316 440 A1 or the corresponding application DE 26 28 807 A1.
According to U.S. Pat. No. 5,743,708 A, an impingement plate may rest on a steps of a nozzle segment. For each aerofoil a separate nozzle segment seems to be required. A plurality of impingement plates are provided for each nozzle segment to individually be placed in a plurality of compartments. The compartments are separated by internal railings that have openings to be in fluid communication with one another. The rim of the aerofoil fluid inlet or fluid outlet is elevated such that the inlet projects over the impingement plates and such that small through holes are present through the rim to allow impingement fluid from the compartments to enter the hollow aerofoil. It is apparent that a large number of small sections of impingement plates need to be assembled.
Further turbine airfoil arrangements are known from DE 10 20087 055 574 A1 and EP 1 548 235 A2 which both show turbine airfoil arrangement segments that comprise two aerofoils on a monolithic segment.
It is an object of the invention to provide cooling features for a turbine nozzle segment such that cooling of aerofoils and platforms will happen reliably. Furthermore it is an additional goal to have a fairly simple design which is easy to be assembled.